One of the most common types of locks uses a key to place tumblers within a housing into preferred orientations. This type of lock is commonly referred to as a pin tumbler lock, wherein the tumblers must be oriented in order to operate.
Pin tumbler locks utilize an inner key plug disposed within a lock housing. Insertion of a key into the key plug elevates tumblers which lie in tubes cut into the lock housing and key plug. When the correct key is inserted into the key plug, the tumblers are directed, as a group, into an unlocked orientation. When in this unlocked orientation, the tumblers create a gap or shear line between the key plug and the lock housing. When this gap is present, the lock is unsecured and the key plug will rotate within the lock housing.
Early pin tumbler locks were susceptible to unauthorized entry via a surreptitious process known as "lock picking." Lock picking involves inserting a narrow rod into the keyway of the key plug and rotating the key plug until the tumblers bind against the inside walls of the tumbler tubes. Once bound against the walls of the tumbler tubes, the tumblers may be individually lifted by a key pick and held in place frictionally by their contact with the tube inner walls. Through trial and error, the tumblers are lifted to their preferred location, creating the rotation-allowing gap or shear line within the lock. This approach simulates the presence of a key and allows the lock to be opened.
Locks that are pick-resistant, have modified tumblers. Placing these modified tumblers into their correct positions required lifting and twisting. This lifting and twisting was accomplished by keys with angled cuts that engaged inclined bottom surfaces of the tumblers. These special keys created a shear gap that allowed key plug rotation within the lock housing.
Although these improved pin tumbler locks were pick resistant, they had other problems. For example, these designs were susceptible to methods of forced entry known as "drilling a lock." A drill could be used to cut through the front of a lock, forcing the drill bit against the tumblers or other vital elements within the lock, and destroying the key plug-securing elements of the lock. Once these vital elements were destroyed, the key plug would rotate within the lock housing. "Drilling a lock" eliminated the need for keys or lock picking tools.
Additional lock designs were created to guard against this "lock drilling" procedure. Hardened-steel inserts prevented a drill bit from destroying vital securing elements inside the lock. Unfortunately, these known reinforcements were irrelevant in the face of other forced entry methods. As described below, known reinforced locks were still vulnerable.
Pin tumbler locks typically include a tailpiece that links the key plug with a latch operating bar. The tailpiece extends from the key plug and rotates when the key plug rotates. When the tailpiece is rotated, it engages a contoured aperture disposed within the latch operating bar. When the correct key is inserted into the key plug, the key, key plug, and tailpiece rotate as a unit, from a first, key-insertion position to a second, locking position.
As the inserted key is turned, the tailpiece rotates to a second, locking position. During this rotation, the tailpiece engages ramped teeth in the latch operating bar aperture. By engaging the ramped teeth, the rotating tailpiece imparts lateral motion to the latch operating bar. A securing latch, commonly referred to as a "deadbolt", is attached to the latch bar and moves into a locking position because of this lateral motion.
Rotation of the key from this second, locking position back to the first position will return the tailpiece and key plug to the first position. However, because of the ramped shape of the teeth, this second-position-to-first-position rotation does not produce any tailpiece-tooth engagement. As a result, although the tailpiece and key plug move from the second position to the first position, the latch and latch operating bar remain extended in a locking position. In other words, the tailpiece "slips" within the aperture of the latch operating bar when the tailpiece moves from the second position back to the first position. It is only when the key plug and tailpiece are rotated from the second position, past the first position, and into a third position that the latch is moved from the extended or locked position into a recessed or unlocked position. This is because the tailpiece engages the inner teeth of the latch operating bar aperture when it moves from the second position through to the third position.
Since the tailpiece and latch operating bar are not rigidly linked, only certain motions of one will move the other. This "limited-slip" arrangement allows an individual to remove a key from this type of lock without placing the lock into an unsecured orientation. Unfortunately, this limited slippage also makes the lock vulnerable to forced entry.
Known pin tumbler locks are pick-resistant and even include features to prevent destruction of vital elements within the lock. These reinforcements are designed to prevent the key plug from rotating within the lock housing. This, in turn, prevents motion of the tailpiece and subsequent motion of the latch.
However, the limited slip that allows the tailpiece to move without disturbing the latch conversely allows the latch to move without engaging the tailpiece. Thus, it is possible to move the latch operating bar without moving the tailpiece. The latch operating bar and the attached securing latch may be translated or slid from an extended, door-securing position to a retracted, door-releasing position without binding against the locker key plug and tailpiece. A secured tailpiece, therefore, does not guarantee a secured latch.
While known lock designs prevent unwanted motion of key plugs and tailpieces, they do not prevent the latch operating bar from moving separately from the tailpiece. This is troublesome because although known locks include inserts to prevent drilling aimed at destroying cylinder securing elements of the lock, known locks do not stop attacks designed to move the latch directly without using the tailpiece.
Drilling through the relatively-soft metal of the key plug and lock housing will expose a pathway to the aperture in the latch operating bar. A screwdriver, or the drill bit used to create the tunnel itself, will imitate the tailpiece if placed within the latch operating bar aperture. When rotated, the inserted screwdriver or drill bit will provide the appropriate twisting motion needed to operate the latch. The latch may be slid, without a key, from its locked position to its unlocked position, allowing a previously-secured door to open. All of this motion is possible without disturbing the lock tumblers or other securing elements within the lock.
This "latch manipulating" method of forced entry completely circumvents known lock reinforcements. Known locks prevent unwanted motion of the key plug and tailpiece as a way preventing latch motion. Unfortunately, these designs are vulnerable to methods of forced entry that move the latch directly, without relying on tailpiece motion. Since the latch will move even if the tailpiece will not, after a path to the latch operating bar has been drilled, a simple twist of a screwdriver will move the latch and open the lock. However, a screwdriver is often not needed: if the rotating drill bit contacts the aperture teeth during drilling, the bit itself moves the latch operating bar and opens the lock.
Creating locks made entirely from drill-resistant materials is not practical. Locks are typically installed with mounting screws that pass through the lock housing. As a result, the lock housing includes threaded bores to accommodate the mounting screws. Since the threaded bores must be cut into the housing, making the lock from drill-resistant material would unduly hamper the bore-creating process.
Creating locks made entirely from drill-resistant materials is also not desirable: drill-resistant materials are often brittle by nature. Locks constructed from excessive amounts of drill-resistant material may trade one problem for another: brittle materials often shatter under direct impact. Locks of the prior art teachings made with excessive amounts of drill-resistant materials may be destroyed by blows from a hammer, for example. As such, no key or drilling is necessary.
Thus, what is needed in this art is a device which prevents unwanted access to the latch operating bar. Furthermore, the device should resist forced entry attempts that open the lock by moving the latch operating bar directly, without tailpiece motion.